The One Ring

It is described in The Hobbit as a magic ring of invisibility. In The Lord of The Rings, Tolkien ascribes to the Ring a darker character, with malevolent power going far beyond conferring invisibility: it was created by Sauron the Dark Lord as part of his design to win domination over Middle-earth.

The Lord of the Rings is about the quest to destroy the One Ring and keep Sauron from fulfilling his malevolent design.